System including a first inverter and a second inverter, and method for operating the system

ABSTRACT

In a system and method for operating a system, which includes a first inverter which feeds a first electric motor, and a second inverter which feeds a second electric motor, the DC-voltage side connection of the first inverter is connected to the DC-voltage side connection of a rectifier which is supplied from an electrical AC-voltage supply network, the DC-voltage side connection of the second inverter is connected to the DC-voltage side connection of the rectifier, in particular, the two DC-voltage side connections of the inverters are switched in parallel, and a controller is provided, in particular in the first inverter, which controls the current accepted and acquired by the first inverter at its DC-voltage side terminal toward a setpoint value in that the torque of the first electric motor fed by the first inverter is the controlled variable.

FIELD OF THE INVENTION

The present invention relates to a system, which includes a firstinverter and a second inverter, and to a method for operating thesystem.

BACKGROUND INFORMATION

An inverter feeds an electric motor and can be supplied by a rectifierat its DC-voltage side connection.

German Patent Document No. 10 2006 033 562 describes a servo pressincluding an energy management.

German Patent Document No. 10 2010 023 536 describes a device and amethod for an intelligent drive-based network power control with the aidof a kinetic energy storage.

European Patent Document No. 2 525 481 describes a control device for anindirect converter as well as an indirect converter itself.

German Patent Document No. 101 50 347 describes a heavy fan forthree-phase motors in passenger transportation systems.

SUMMARY

Example embodiments of the present invention provide a system in whichthe lowest possible power withdrawal from an AC-voltage supply networkis to be required.

According to an example embodiment of the present invention, a systemincludes a rectifier which is able to be supplied from an electricalAC-voltage supply network, a first inverter which feeds a first electricmotor, and a second inverter which feeds a second electric motor. TheDC-voltage side connections of the first and the second inverter areconnected to the DC-voltage side connection of the rectifier in eachcase, and a current acquisition device is situated inside the housing ofthe first inverter. Either the current acquisition device is adapted toacquire the current emerging from the rectifier at the DC-voltage sideconnection of the rectifier, or the current acquisition device isadapted to acquire the currents entering from the electrical AC-voltagesupply network at the AC-voltage side connection of the rectifier. Asupply of the network-side three-phase current is possible on thehousing of the first inverter, which is conveyed to the currentacquisition device and is routed out of the housing of the firstinverter to the housing of the rectifier.

This offers the advantage that the current acquisition may be conveyeddirectly to the signal electronics of the transformer, which thereforemeans that no energy-intensive conversion of the signals has to becarried out.

According to an example embodiment of the present invention, a systemincludes a rectifier which is able to be supplied from an electricalAC-voltage supply network, a first inverter which feeds a first electricmotor, and a second inverter which feeds a second electric motor. TheDC-voltage side connection of the first inverter is connected to theDC-voltage side connection of the rectifier, and the DC-voltage sideconnection of the second inverter is connected to the DC-voltage sideconnection of the rectifier, in particular, the two DC-voltage sideconnections of the inverters are switched in parallel. A controller unitis provided, in particular in the first inverter, which controls thepower accepted from the AC-voltage supply network, in particular via therectifier, toward a setpoint value by setting the torque of the electricmotor fed by the first inverter, the controller unit particularly havinga feed-forward path so that the power output or accepted by the secondelectric motor via the second inverter is effective as the feed-forwardsignal, or which controls the acquired current that emerges at theDC-voltage side connection of the rectifier toward a setpoint value inthat the torque of the electric motor fed by the first inverter is thecontrolled variable, and the setpoint value is a measure of the poweraccepted from the network and is constant, in particular.

This offers the advantage that as little power as possible is withdrawnfrom the AC-voltage supply network. Thus, in an operation of the secondelectric motor during which a constant switch takes place between agenerator mode and a motor mode, the first drive is used as a flywheelenergy store, which therefore means that less power is received from theAC-voltage supply network via the rectifier.

In example embodiments, a current acquisition device adapted to acquirethe current that emerges at the DC-voltage side connection of therectifier determines a value I_ZK_N and this value is conveyed to aproduct generation device, which multiplies the value by the acquiredvalue of voltage U_ZK_actual applied at the DC-voltage side connectionof the rectifier. This is considered advantageous insofar as it is easyto determine the power withdrawn from the AC-voltage supply network.

In example embodiments, the controller unit includes a linear controllerand the feed-forward path, the difference between the acquired ordetermined power accepted from the AC-voltage supply network, inparticular the power accepted via the rectifier, and a setpoint value ofthe power received from the AC-voltage supply network is conveyed to thecontroller as the input signal, the feed-forward signal is added to theoutput signal of the linear controller and a signal that corresponds toa power loss of the first electric motor is added to the sum signalgenerated in this manner, and the thereby generated signal is conveyedto a division device, which supplies the signal that is divided by asignal corresponding to the rotational frequency of the first electricmotor as the output signal, the output signal is used as the controlledvariable and corresponds to the torque of the electric motor fed by thefirst inverter. This has the advantage that a rapid adjustment controlis achievable with the aid of the feed-forward control. In addition, arapid adjustment control is also achievable by considering the powerloss of the first motor. The torque as the controlled variable is easilydetermined with the aid of the division by the angular velocity of themotor, in particular the rotational frequency.

In example embodiments, the linear controller is a PI controller. Thisis considered advantageous insofar as a simple configuration of thecontroller is able to be provided.

In example embodiments, the signal corresponding to the rotationalfrequency of the first electric motor is acquired with the aid of asensor for sensing the rotational frequency or position, which isdisposed on the first electric motor, or it is determined on the basisof the acquired value of the motor current of the first electric motorand on the basis of the value of the acquired motor voltage or thevoltage acquired at the DC-voltage side connection of the rectifier, inparticular with the aid of a machine model. This offers the advantagethat the acquired rotational frequency may be used for the most currentand accurate determination of the controlled variable.

In example embodiments, the power accepted by the second electric motorvia the second inverter is formed by the voltage acquired at theDC-voltage side connection of the rectifier and the current accepted andacquired by the second inverter. This is considered advantageous insofaras the power of the second motor is easily determined.

In example embodiments, the power accepted by the first electric motorvia the first inverter is formed by the voltage acquired at theDC-voltage side connection of the rectifier and the current accepted andacquired by the first inverter. This has the advantage that the power ofthe first motor is easily determined.

According to an example embodiment of the present invention, a methodfor operating a system that includes a rectifier which is able to besupplied from an electrical AC-voltage supply network, a first inverterwhich feeds a first electric motor, and a second inverter which feeds asecond electric motor, in particular, the system is arranged in theafore-described manner, the DC-voltage side connection of the firstinverter is connected to the DC-voltage side connection of therectifier, the DC-voltage side connection of the second inverter isconnected to the DC-voltage side connection of the rectifier, inparticular, the two DC-voltage side connections of the inverters areswitched in parallel, in particular, a controller unit is provided, inparticular in the first inverter, and the power accepted from theAC-voltage supply network, in particular via the rectifier, iscontrolled towards a setpoint value by setting the torque of theelectric motor supplied by the first inverter, and the power output oraccepted by the second electric motor via the second inverter iseffective as the feed-forward signal.

This offers the advantage that the power withdrawn from the AC-voltagesupply network is easily able to be reduced to the smallest valuepossible.

In example embodiments, the first electric motor is developed as a fanmotor, and the flow of cool air supplied by the fan motor is guidedalong the second electric motor and/or the second inverter and/orconducted through the second electric motor, or in other words, a fan isparticularly connected to the rotor shaft of the first electric motor ina torsionally fixed manner. This offers the advantage that the fan maybe used as a flywheel mass. Thus, the fan motor is able to be operatedas a flywheel energy store.

In example embodiments, when the second electric motor is operated in agenerator mode, the flow of cool air increases and thus becomes greater,in particular. This offers the advantage that it is possible to supplypower to the flywheel energy store in a generator-mode operation,meaning that the flywheel energy store arranged as a fan motor acceptsmore energy. Due to the higher rotational frequency, a stronger airflowis therefore generated and the second inverter and/or the secondelectric motor is/are cooled more heavily.

In example embodiments, a difference generation device determines thesetpoint value (I_ZK_SMS_setpoint) by generating the difference betweena constant value (const) and the acquired value (I_ZK_App) of thecurrent accepted by the second inverter at its direct current sideconnection. This offers the advantage that the constant valuecorresponds to the current withdrawal from the AC-voltage supplynetwork. Thus, the excess current is withdrawn from the flywheel energystore or supplied to it.

In example embodiments, the constant value (const) is as small aspossible, in particular in the range that is averaged over an extendedperiod of time. This is considered advantageous insofar as a minimalcurrent withdrawal from the AC-voltage supply network over the longestperiod of time possible may be achieved. The duration of the time perioddepends on the energy capacity limits of the flywheel energy store. Forexample, a further energy withdrawal from a discharged energy store,i.e. flywheel energy store, is unable to take place. In the same manner,given a maximally permissible charge, a further supply of power to theenergy store is no longer allowed.

In example embodiments, the housing of the first inverter also surroundsthe second inverter in a housing-forming manner. This is consideredadvantageous insofar as the most compact configuration possible isachievable. In addition, a single signal electronics, for example, issufficient for both inverters.

In example embodiments, a signal electronics generates the controlsignals for the power semiconductor switches of the first inverterdisposed in half-bridges and generates the control signals for the powersemiconductor switches of the second inverter disposed in half-bridges.This is considered advantageous insofar as a data bus is omitted and asingle signal electronics thus generates the control signals of bothinverters.

In example embodiments, a signal electronics of the first inverter isconnected to the signal electronics of the second inverter with the aidof a data bus for a data exchange. This is considered advantageousinsofar as the two signal electronics of the two inverters are able tobe operated in a synchronized manner with the aid of the data bus. Thetwo inverters may be placed in separate housings.

In example embodiments, a difference formation device determines acontroller input value by generating the difference between a constantvalue (I_ZK_N_setpoint) and the value (I_ZK_N), acquired by the firstinverter, of the entire current supplied by the rectifier to theintermediate circuit. The controller which adjusts this difference hasthe setpoint torque of the first motor as the controlled variable. Thisoffers the advantage that the constant value corresponds to the currentwithdrawal from the AC-voltage supply network, and the excess current istherefore withdrawn from the flywheel energy store or supplied thereto.

In example embodiments, the constant value (I_ZK_N_setpoint) is as lowas possible, in particular in the range that is averaged across anextended period of time. This has the advantage that the longestpossible minimal current withdrawal from the AC-voltage supply networkis achievable. The duration of the period depends on the energy capacitylimits of the flywheel energy store. For example, a further energywithdrawal from a discharged energy store, i.e. flywheel energy store,is unable to take place. In the same manner, with a maximally permittedcharge, a further supply of power to the energy store is no longerallowed. In the standard case, the minimal current withdrawal, and thusa minimal power acceptance from the network, is achieved on a continuousbasis.

In example embodiments, a further measuring device is provided, whichacquires the current of the second inverter. (I_ZK_App). This offers theadvantage that it is possible to determine the power of the secondinverter required for the feed-forward control with the aid of theacquired current of the second inverter, this being done by multiplyingthe current of the second inverter by the measured intermediate circuitvoltage U_ZK_actual. In the case of more than one application inverter,I_ZK_App corresponds to the sum current of all application inverters.

Further features and aspects of example embodiments of the presentinvention are described in greater detail below with reference to theappended Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In FIG. 1, a system according to an example embodiment of the presentinvention is shown.

In FIG. 2, a control method used in the system is schematicallyillustrated.

In FIG. 3, a further control method is schematically illustrated.

DETAILED DESCRIPTION

As illustrated in FIG. 1, a rectifier 2, at whose DC-voltage side outputtwo inverters (3, 4) are connected which are switched in parallel, issupplied from an AC-voltage supply network 1.

The DC-voltage side output of rectifier 2, i.e. the intermediatecircuit, has voltage U_ZK.

Current I_ZK_N emerging or entering at the DC-voltage side connection ofrectifier 2 is acquired. The current withdrawn by second inverter 3 isdenoted by I_ZK_App, and the current withdrawn by first inverter 4 iscorrespondingly denoted by I_ZK_SMS.

The current acquisition device, i.e. for acquiring current I_ZK_N, isdisposed inside the housing of first inverter 4. The acquired currentvalues are thus conveyed to the signal electronics of the first inverterwithout long lines, i.e. directly.

As an alternative, current I_ZK_N may also be determined from thecurrents, in particular phase currents (I_(R), I_(S), I_(T)), of theAC-voltage supply network acquired at the AC-voltage side connection ofrectifier 2. This requires at least two acquired phase-current values(I_(R), I_(T)). The phase-current acquisition device, i.e. for theacquisition of the phase currents (I_(R), I_(S), I_(T)), is locatedinside the housing of first inverter 4. Thus, the acquired currentvalues are conveyed to the signal electronics of the first inverterwithout long lines, i.e. directly.

As illustrated in FIG. 1, second inverter 3 feeds a three-phase motor 5,which is connected to an AC-voltage side output of second inverter 3.Second inverter 3 has a power output stage made up of threehalf-bridges, and a power semiconductor switch, in particular an IGBT ora MOSFET, is disposed in each upper and lower branch of eachhalf-bridge. The power semiconductor switches of this power output stageof inverter 3 are controlled with the aid of pulse-width-modulatedsignals in each case, these signals being generated by a signalelectronics of second inverter 3. The signals from the signalelectronics of second inverter 3 are generated such that the motorcurrent is controlled toward a predefined setpoint value via the motorvoltage set by second inverter 3. A setpoint value for torquecorresponds to the setpoint value of the motor current. Alternatively,it is also possible to control toward a setpoint value for therotational frequency.

Intermediate circuit voltage U_ZK is also applied at the DC-voltage sideconnection of first inverter 4, which supplies an electric motor of aflywheel energy store. The electric motor may be arranged as athree-phase motor whose motor phase currents are denoted by I_U, I_V andI_W. The three-phase motor may be arranged as an asynchronous motor. Theflywheel mass may be arranged a fan which is connected to the rotor ofthe electric motor in a torsionally fixed manner so that the fan motorfunctions as a flywheel energy store.

In other words, the electric motor has a rotor shaft which is connectedin a torsionally fixed manner to the rotatably mounted flywheel mass 6,which may be arranged in the form of a fan, so that, in the motoroperation of the motor, energy is able to be stored in the flywheelenergy store from the direction of the electric motor and withdrawn in agenerator-mode operation.

First inverter 4 thus feeds a three-phase motor which is connected tothe AC-voltage side output of first inverter 4. First inverter 4 alsohas a power output stage which is made up of three half-bridges, and apower semiconductor switch, in particular an IGBT or MOSFET, is disposedin each upper and lower branch of each half-bridge. The powersemiconductor switches of this power output stage of first inverter 4are controlled with the aid of pulse-width-modulated signals, which aregenerated by a signal electronics of first inverter 4. The signalselectronics of first inverter 4 generates the signals such that themotor current is controlled toward a predefined setpoint value throughthe motor voltage set by first inverter 4. A setpoint value for torquecorresponds to the setpoint value of the motor current. Alternatively,it is also possible to control toward a setpoint value characteristic ofthe rotational frequency.

As illustrated in FIG. 1, the rectifier 2 may be enclosed and/orsurrounded by a housing 10 of the first inverter 4 in a housing-formingmanner.

Thus, according to an example embodiment of the present invention, asillustrated in FIG. 1, for example, a system includes: a rectifier 2adapted to be supplied from an electrical AC-voltage supply network 1; afirst inverter 4 adapted to feed a first electric motor; a secondinverter 3 adapted feed a second electric motor 5; and acurrent-acquisition device 11 arranged inside a housing 12 of the firstinverter 4. DC-voltage side connections of the first and the secondinverter 4, 3 are connected to a DC-voltage side connection of therectifier 2. The current-acquisition device 11 is adapted to acquirecurrent emerging from the rectifier 2 at the DC-voltage side connectionof the rectifier 2 and/or to acquire currents entering from theelectrical AC-voltage supply network 1 at an AC-voltage side connectionof the rectifier. The housing 12 of the first inverter 4 is adapted forsupply from a network-side three-phase current, to convey thenetwork-side three-phase current to the current acquisition device 11,and/or to route the network-side three-phase current out of the housing12 of the first inverter 4 to a housing 13 of the rectifier 2.

According to example embodiments of the present invention, the controlmethod shown in FIG. 2 is applied.

For this purpose, current I_ZK_N emerging at the DC-voltage sideconnection of rectifier 2 is acquired and controlled toward a setpointvalue I_ZK_N_setpoint by setting the torque of motor DR of flywheelenergy store 9.

As an alternative, instead of acquiring current I_ZK_N, current I_ZK_SMSentering or emerging at the DC-voltage side connection of the firstinverter is acquired, and the current I_ZK_App entering or emerging atthe DC-voltage side connection of the second inverter is acquired. Usingthese two acquired values, it is possible to determine current I_ZK_Nwith the aid of a summing operation.

Due to the control schematically illustrated in FIG. 2, the currentsupplied by rectifier 2, i.e. by the AC-voltage supply network, does notexceed a constant value const, or at least does not exceed it on acontinuous basis. This is because current I_ZK_SMS accepted or output byflywheel energy store 9 is correspondingly controlled in an indirectmanner.

As illustrated in FIG. 2, the difference between setpoint valueI_ZK_N_setpoint and actual value I_ZK_N actual is forwarded to thecontroller. The controller is implemented as a linear controller, inparticular as a PI controller whose controlled variable is the setpointvalue of torque M_setpoint of electric motor DR of flywheel energy store9 fed by inverter 4. Thus, electric motor DR of flywheel energy store 9is accelerated or decelerated such that the sum of its own load valueand the transmitted load value of second drive 8, in particular ofelectric motor 5, has a constant value, in particular the smallest valuepossible, or is at least controlled toward such a value.

As a result, the current withdrawal from rectifier 2 induced by electricmotor 5 via inverter 3 is able to be restricted in that flywheel energystore 9 supplies a corresponding current component.

As illustrated in FIG. 1, a fan is mounted in a torsionally fixed manneron the rotor connected to flywheel mass 6, i.e. in particular alsoconnected to the rotor in a torsionally fixed manner. In this manner,the airflow supplied by the fan is used for cooling drive 8, inparticular of motor 5 and/or inverter 3 which supply or supplies it.

As an alternative, flywheel mass 6 is able to be obtained through acorresponding mass-rich configuration of the fan. As a result, a fandriven by a motor is able to be operated as a flywheel energy store. Theuse of the control shown in FIG. 2 in such a fan motor may also beutilized when the maximum power of drive 8 is less than the powermaximally able to be accepted or output by the fan motor.

Alternatively, the network power may be determined either approximatelyby multiplying I_ZK_N by the intermediate circuit voltage U_ZK, ordirectly by measuring the three phase currents and the three phasevoltages. Because the characteristic of the feeding network (symmetry,root-mean-square value of the voltage) is known, fewer than thementioned six phase variables may also be sufficient for determining thenetwork power. If the network power is known, then it is able to becontrolled to a minimum.

As illustrated in FIG. 3, the actual value of power P_ZK introduced intothe intermediate circuit from the AC-voltage supply network viarectifier 2 is determined in that the product of the actual valueU_ZK_actual of the intermediate circuit voltage, acquired using acorresponding voltage-acquisition device, and the actual value ofcurrent I_ZK_N, acquired or determined from I_ZK_App and I_ZK_SMS.Alternatively, the network power may be determined either approximatelyby multiplying I_.Z_K_N by intermediate circuit voltage U_ZK, ordirectly by measuring the three phase currents and the three phasevoltages. Because the characteristic of the feeding network (symmetry,root-mean-square value of the voltage) is known, fewer than thementioned six phase variables may also be sufficient for determining thenetwork power. If the network power is known, it is able to becontrolled to a minimum.

This power P_ZK is controlled toward a setpoint value P_ZK_setpoint byforwarding the difference between P_ZK and P_ZK_setpoint to acontroller, in particular to a linear controller, the controllergenerating a controlled value to which the power P_App accepted by thesecond inverter 3, and thus particularly also by second motor 5, isadded as a feed-forward value, the result of this summation beingdenoted by P_reg.

P_App is determined as the product of the acquired actual valueU_ZK_actual and the actual value of current I_ZK_App accepted by thesecond inverter at its direct-current side connection.

In addition, a value P_V_SMS is added to determined value P_reg so thata controlled value P control is determined, in which power loss P_V_SMSof the flywheel energy store has been taken into account.

Dividing controlled value P_control by the rotational frequency Ω_SMS,i.e. in particular the angular velocity, of the rotor of the electricmotor of the flywheel energy store makes it possible to determinecontrolled value M_setpoint therefrom, i.e. the desired torque ofelectric motor DR of the flywheel energy store.

Thus, it is important that a linear controller controls the power of thedrive of the flywheel energy store toward a setpoint value by settingthe torque of the drive of the flywheel energy store appropriately, forwhich the power accepted by the second inverter is used as thefeed-forward value.

In a generator-mode operation, three-phase motor 5 is cooled moreheavily, or especially heavily, by flywheel energy store 9 functioningas a fan because power is discharged to flywheel energy store 9, whichmeans that its rotational frequency increases.

If the current acquisition device for acquiring the current entering atthe DC-voltage side connection of the rectifier is situated in thehousing of first inverter 4, i.e. in particular the device for acquiringthe phase currents, then a supply of the network-side three-phasecurrent at the housing of the first inverter is possible, which issupplied to the current acquisition device and is routed out of there tothe housing of the rectifier. In addition, the DC-voltage sideconnection of rectifier 2 is connected to a corresponding DC-voltageside connection, i.e. the intermediate circuit connection, of firstinverter 4. Moreover, the first motor is electrically connected to theDC-voltage side connection of first inverter 4, in particular with theaid of a three-phase cable.

In example embodiments, the signal electronics of second inverter 3 isconnected to the signal electronics of first inverter 4 for a dataexchange with the aid of a field bus.

LIST OF REFERENCE CHARACTERS

-   1 AC-voltage supply network-   2 rectifier-   3 inverter of second drive 8-   4 inverter of first drive 7-   5 three-phase motor-   6 flywheel mass-   7 first converter, in particular an inverter-   8 first drive-   9 flywheel energy store-   10 housing-   11 current-acquisition device-   12 housing of first inverter-   13 housing of rectifier-   CPU signal electronics including processing unit-   DR three-phase motor, in particular an asynchronous motor-   U_ZK intermediate circuit voltage-   U_ZK_Ist acquired actual value of the intermediate circuit voltage-   P_Netz_Soll setpoint value of the power received from the AC-voltage    supply network-   P_ZK power received from the AC-voltage supply network-   P_App power of the second drive-   P_reg idealized controlled value of the power-   P_V_SMS power loss of the flywheel energy store, especially power    loss as a function of the rotational frequency-   P_Stell controlled value of the power of the first drive-   Ω_SMS rotational frequency of the rotor of the electric motor of the    flywheel energy store-   I_ZK_N current transmitted from rectifier 2 to the intermediate    circuit-   I_ZK_N_Soll setpoint value for current I_ZK_N-   I_ZK_App actual value of the input current of inverter 3-   I_ZK_SMS actual value of the input current of inverter 4-   I_ZK_SMS_Soll setpoint value of the input current of inverter 4-   I_U first phase current of motor DR-   I_V second phase current of motor DR-   I_W third phase current of motor DR-   M_Soll torque setpoint value-   Const setpoint current, in particular setpoint current accepted by    rectifier 2 into the intermediate circuit-   K_P proportionality component-   K_I integral component

The invention claimed is:
 1. A system, comprising: a rectifier adaptedto be supplied from an electrical AC-voltage supply network; a firstinverter adapted to feed a first electric motor; a second inverteradapted feed a second electric motor; and a current-acquisition devicearranged inside a housing of the first inverter; wherein DC-voltage sideconnections of the first and the second inverter are connected to aDC-voltage side connection of the rectifier; wherein thecurrent-acquisition device is adapted to acquire current emerging fromthe rectifier at the DC-voltage side connection of the rectifier and/orto acquire currents entering from the electrical AC-voltage supplynetwork at an AC-voltage side connection of the rectifier, and whereinthe housing of the first inverter is adapted for supply from anetwork-side three-phase current, to convey the network-side three-phasecurrent to the current acquisition device, and/or to route thenetwork-side three-phase current out of the housing of the firstinverter to a housing of the rectifier.
 2. A system, comprising: arectifier adapted to be supplied from an electrical AC-voltage supplynetwork; a first inverter adapted to feed a first electric motor; acontroller unit provided in the first inverter; and a second inverteradapted to feed a second electric motor; wherein a DC-voltage sideconnection of the first inverter is connected to a DC-voltage sideconnection of the rectifier, a DC-voltage side connection of the secondinverter is connected to the DC-voltage side connection of therectifier, the DC-voltage side connections of the inverters beingswitched in parallel; wherein the controller unit is: (a) adapted tocontrol power accepted from the AC-voltage supply network, via therectifier, toward a setpoint value by setting a torque of the firstelectric motor, the controller unit including a feed-forward path sothat power output or accepted by the second electric motor via thesecond inverter becomes effective as a feed-forward signal, and/or (b)adapted to control an acquired current emerging at the DC-voltage sideconnection of the rectifier toward a setpoint value in that the torqueof the first electric motor is a controlled variable, and the setpointvalue is a measure of power accepted from the network and beingconstant.
 3. The system according to claim 2, wherein a currentacquisition device adapted to acquire current emerging at the DC-voltageside connection of the rectifier is adapted to determine a value and toconvey the value to a product generation device adapted to multiply thevalue by an acquired value of the voltage applied at the DC-voltage sideconnection of the rectifier.
 4. The system according to claim 2, whereinthe setpoint value is as low as possible and/or is as low as possible ina range that is averaged over an extended period of time.
 5. The systemaccording to claim 2, wherein the rectifier is arranged as a structuralpart of the first inverter, and the rectifier is enclosed and/orsurrounded by a housing of the first inverter in a housing-formingmanner.
 6. The system according to claim 2, wherein the controller unithas a linear controller and the feed-forward path, a difference betweenacquired or determined power accepted from the AC-voltage supply networkand/or power accepted via the rectifier, and a setpoint value of thepower received from the AC voltage-supply network is conveyed to thelinear controller as an input signal, and the feed-forward signal isadded to an output signal of the linear controller and a signal thatcorresponds to a power loss of the first electric motor is added to asum signal generated in this manner, and the thereby generated signal isconveyed to a division device adapted to supply the thereby formedsignal divided by a signal corresponding to a rotational frequency ofthe first electric motor as an output signal adapted for use as acontrolled variable and corresponding to a torque of the first electricmotor.
 7. The system according to claim 6, wherein the linear controllerincludes a PI controller.
 8. The system according to claim 6, whereinthe signal corresponding to the rotational frequency of the firstelectric motor is acquired with the aid of a sensor provided on thefirst electric motor, and/or is determined from an acquired value of amotor current of the first electric motor and from value of an acquiredmotor voltage or a voltage acquired at the DC-voltage side connection ofthe rectifier, with the aid of a machine model.
 9. The system accordingto claim 2, wherein power accepted by the second electric motor via thesecond inverter is formed by a voltage acquired at the DC-voltage sideconnection of the rectifier and a current accepted and acquired by thesecond inverter.
 10. The system according to claim 2, wherein poweraccepted by the first electric motor via the first inverter is formed bya voltage acquired at the DC-voltage side connection of the rectifierand current accepted and acquired by the first inverter.
 11. The systemaccording to claim 2, wherein the first electric motor is arranged as afan motor adapted to supply a flow of cool air that is conveyed alongthe second electric motor and/or the second inverter and/or through thesecond electric motor.
 12. The system according to claim 11, wherein afan is connected in a torsionally fixed manner to a rotor shaft of thefirst electric motor.
 13. The system according to claim 11, wherein in agenerator-mode operation of the second electric motor, the flow of coolair increases, and thus becomes larger.
 14. The system according toclaim 2, wherein signal electronics are adapted to generate controlsignals for power semiconductor switches of the first inverter disposedin half-bridges and to generate control signals for power semiconductorswitches of the second inverter disposed in half-bridges.
 15. The systemaccording to claim 2, wherein signal electronics of the first inverterare connected via a data bus to signal electronics of the secondinverter for a data exchange.
 16. A method for operating a system thatincludes a rectifier adapted to be supplied from an electricalAC-voltage supply network, a first inverter adapted to feed a firstelectric motor and including a controller unit, and a second inverteradapted to feed a second electric motor, a DC-voltage side connection ofthe first inverter being connected to a DC-voltage side connection ofthe rectifier, a DC-voltage side connection of the second inverter beingconnected to the DC-voltage side connection of the rectifier, theDC-voltage side connections of the inverters being switched in parallel,comprising: controlling power accepted from the AC-voltage supplynetwork, via the rectifier, toward a setpoint value by setting a torqueof the first electric motor, power output and/or accepted by the secondelectric motor via the second inverter being effective as a feed-forwardsignal.